1. Field of the Invention
The present invention relates to a photo detector device for converting optical energy into an electric signal, and particularly to a transparent photo detector device.
2. Description of the Prior Art
Many photo detector devices have been developed for converting light such as visible light or infrared light into an electric signal to obtain an electric signal corresponding to the amount of light. Particularly, a photo detector device using amorphous silicon type semiconductor, which is sensitive to the visible light, as a light sensor element can be formed on a glass substrate, so that it enables an increased semiconductor area, a reduced cost, and simplified manufacturing process. In addition, it has an advantage that it can be finely worked to obtain any pattern by using, e.g., photolithography during the manufacturing process, so that the application field thereof is being enlarged. For example, there are an automatic focus detector device for a camera and a reading device for a facsimile disclosed in the Patent Laying-Open Gazette No. 167002/1982 and the Patent Publication Gazette No. 14073/1983.
A conventional light sensor element is illustrated in FIGS. 1A, 1B and 1C. FIG. 1A is a rear perspective view of the sensor element; FIG. 1B is a cross sectional view taken along the line 1B--1B in FIG. 1A; and FIG. 1C is a cross sectional view taken along the line 1C--1C in FIG. 1A. Referring to these figures, a light sensor element LS is formed of a transparent front electrode 3 on a main surface la of a transparent insulator substrate 1, a semiconductor layer 4 for photoelectric function, and a back electrode 5 layered in that order. The semiconductor layer 4 has portions 4a, 4b, 4c and 4d protracting beyond the overlapping region of the front electrode 3 and the back electrode 5.
Referring to FIG. 1B, the protracting portion 4d is indispensable for preventing a short circuit between a side surface 3s of the front electrode 3 and an extended portion 5s of the back electrode 5. However, the effectively functioning region of the sensor element LS is the region where the front electrode 3, the semiconductor layer 4 and the back electrode 5 overlap with one another. Accordingly, the other protruding portions 4a, 4b and 4c are essentially unnecessary. In the transparent photo detector device having this kind of light sensor elements LS dispersively disposed in an one-dimensional or two-dimensional manner, the protruding portions 4a, 4b and 4c are desired to be as small as possible in order to avoid decrease of the transparent region.
In an automatic focus detector device for a camera or the like, a number of light sensor elements must be arranged on a plane for properly sampling high frequency component in the brightness of an image of an object. In case where a number of light sensor elements are arranged in a transparent photo detector device, however, an area of each light sensor element must be smaller in order to prevent deterioration of the light transmissibility of the photo detector device. In that case, an output signal becomes small in proportion to the decrease of the area of the sensor element, so that S/N ratio is lowered in case where there is little amount o incident light.
In the transparent photo detector device, the passing light is desired to go straight without being influenced by refraction or scattering. In addition, in a photo detector device having a plurality of light sensor elements, it is desired that a defect in a certain sensor element or a disconnection at a certain point in the wire pattern does not influence other sensor elements.